368 Dr. B. Sanderson on the Excitation of the [June 11, 



The author then extends his experiments to lead, a metal known to 

 contract greatly in solidifying, and, with respect to which, no one has sug- 

 gested that it expands at the moment of consolidation. He finds that pieces 

 of lead having a specific gravity of 11-361, and being at 70 E., float or 

 sink upon molten lead of the same quality, whose calculated specific 

 gravity was 11*07, according to the relation that subsists between the 

 volume and the " effective " surface of the solid piece, thin pieces with 

 large surface always floating, and vice versa. An explanation is offered 

 of the true cause of the ascending and descending currents observed in 

 very large " ladles " of liquid cast iron, as stated by Messrs. Nasmyth 

 and Carpenter. The facts are shown to be in accordance with those 

 above mentioned, and when rightly interpreted to be at variance with 

 the views of these authors. 



Lastly, the author proceeds to examine the statements made by these 

 writers, as to the floating of lumps of solidified iron furnace-slag upon 

 the same when in a molten state ; he examines the conditions of the 

 alleged facts, and refers to his own experiments upon the total contrac- 

 tion of such slags, made at Barrow Iron-works (a full account of 

 which he has given in his paper on " The true Nature and Origin of 

 Volcanic Heat and Energy," printed in Phil. Trans. 1873), as conclusively 

 proving that such slags are not denser in the molten than in the solid 

 state, and that the floating referred to is due to other causes. The 

 author returns thanks to several persons for facilities liberally afforded 

 him in making these experiments. 



IV. " Note on the Excitation of the Surface of the Cerebral Hemi- 

 spheres by Induced Currents." By J. BURDON SANDERSON, 

 M.D., F.R.S., Professor of Practical Physiology in University 

 College, London. Received April 30, 1874. 



In a paper recently communicated to the E-oyal Society by Dr. Ferrier 

 (Proceedings, No. 151) it is shown that when two ends of copper wire 

 distant from each other not more than a couple of millimetres, and in 

 metallic communication with the terminals of the secondary coil of a 

 Du Bois's induction-apparatus in action, are applied to certain spots of the 

 surface of either hemisphere, and great intensity is given to the induced 

 currents thereby directed through the living tissue, by previously bringing 

 the secondary coil into such a position that it is very close to the primary 

 coil or even partially covers it, characteristic combined movements of the 

 opposite side of the body are produced. 



"With reference to these effects, it was observed by Dr. Eerrier (1) that 

 excitation of the same spot always produces the same movement in the 

 same animal, (2) that the area of excitability for any given movement 

 (or, as it may be called for shortness, the active spot) is extremely small 

 and admits of very accurate definition, and (3) that in different aiiiinals 



